As is well understood in the packaging art, many relatively short shelf-life products, especially dairy products such as cottage cheese, yogurt, sour cream and the like, may be satisfactorily and advantageously packaged in relatively inexpensive, frusto-conically shaped, wide-mouth nestable containers in which the mouth is surrounded by an annular externally projecting curled rim or bead. At one time, two-piece waxed paperboard containers were very widely used for such purposes and in recent years one-piece seamless containers produced from a suitable thermoplastic sheet material by thermoforming have also gained a substantial measure of popularity for such purposes. In any case, any such container must be suitably closed by a separate closure member after the intended product has been packaged therein, and it is important, for economic reasons, that such closure be of such a type that it can be rapidly applied to a filled container, by mechanical closure-applicating equipment, in order to permit the rapid capping of a great number of such containers.
One-piece thermoplastic closures of the recessed rim-locking type are relatively inexpensive and have proven to be functionally satisfactory for the closing of externally rimmed, nestable cottage cheese and other food product containers of the type described above. However, to provide the radially outermost skirt of such a closure with inwardly projecting locking beads of sufficient depth to securely engage the rim of the associated container, it has proven to be quite desirable to form such closures in a female die, where the natural shrinkage of the formed closure, on cooling, will cause the closure to shrink away from the die, thereby facilitating removal of closure from the die in spite of the outwardly projecting locking bead forming portions of the die. In order to get the sheet from which such closure is formed to draw properly into such a die, however, it has been found necessary to maintain a ratio of no more than about 1:1 between the depth of the outer closure skirt and the annular width of the web that extends between such skirt and the peripheral wall which surrounds the recessed central portion of the closure. Because of the substantial width of such web, however, difficulty has been experienced in properly centering the recessed portions of such closures on the open mouths of filled containers when applying such closures mechanically. Likewise, because of the width of such web, heretofore such closures were not always resistant to turning relative to a filled container, when assembled thereto, since the frictional contact between the closure and the container was inherently limited to the contact at the outer skirt of the closure.